The invention relates to steam turbine control systems, more particularly to a control system for an extraction type steam turbine.
A common aspect of many industrial environments is the required simultaneous provision of adequate process steam and electric power. Extraction turbines allow a portion of their inlet steam flow to be directed to a process steam header by use of an extraction valve. They are widely used in industrial environments for cogeneration of process steam and electric power requirements because of their ability to accurately match these requirements in a balanced and stable fashion. In any given industrial plant, these requirements vary over time and an extraction turbine control system attempting to provide and match these requirements must respond accordingly.
Industrial utilization of extraction turbines requires appropriate adjustment of front-end extraction turbine control valves and the extraction valve. These adjustments are made through application of well-known valve position control loop technology.
A control loop is established by a combination of signals, including one representing the desired level of turbine operation, and one representing the existing level of turbine operation. A prior art analog controller functions in the control loop to compare these two signals, and noting any discrepancy, it operates to automatically bring the turbine operation to that level required to balance these signals. The particular combination of signal elements in a control loop reflects the control strategy used by the system designer. The combined operation of several control loops achieves the overall control philosophy used in the control system design.
The majority of extraction turbines in service are used in the industrial area--steel mills, refineries, paper mills, sewage treatment plants, etc., where in the past generation of electricity by the extraction turbine was a byproduct and not really a necessity. The major use of the extraction turbine in these cases was for process steam availability.
In the prior art of extraction turbine control system design, emphasis was placed on control of the process steam extraction operation so as to achieve the extraction process steam pressure required by the industrial plant. Extraction process steam pressure is the important control parameter where the extraction process steam is being used to feed heaters in the plant, such as auxiliary heaters, furnace heaters and building heaters, or where the steam is being used to power steam-driven pumps.
Other uses of extraction process steam include various quenching processes associated with steel mill operations, such as coke-quenching and quenching of hot metal strip as it exits the rolling mill. In these uses, the important control parameter is mass flow of extraction process steam.
For a given extraction steam pipe arrangement, control of either pressure or flow at a specific value necessarily corresponds to a specific value of the other parameter, though uncontrolled. The control scheme for control of either parameter adjusts the extraction valve in accordance with plant requirements. The ability of the control system to switch control modes from a pressure control mode to a flow control mode takes on increasing importance with the expansion in the number of possible ways to utilize the extraction process steam in the industrial process.
Prior art extraction turbine control systems required an operator to perform a complex, lengthy and delicate set-up procedure to accomplish this transfer of control modes. A major difficulty of this set-up procedure was presented by the requirement that it was performed so as to avoid a process upset, that is, that it was bumpless. Therefore, in a transfer from a pressure control mode to a flow control mode, the operator had to establish the flow setpoint at the mass flow value already existing while in the pressure control mode. This required visual comparison of various measurement parameters, introducing the possibility of operator error which would create a large swing in the controlled parameter as the new control mode was entered.
The operator's set-up procedure in all of these cases was further complicated by the need to readjust settings due to the drift introduced by prior art analog control system circuitry which depended on discrete electronic components such as operational amplifiers, capacitors, diodes and resistors, etc. These circuits were prone to drift out of calibration over time and with temperature variations.
With unceasing increases in the costs of energy, personnel and equipment, the inadequacies of older extraction turbine control strategies have become magnified. The potential for operating cost reductions may be available through the application of industrial energy management systems. These optimization systems are arranged to provide the front-end plant boiler controls with the steam pressure, steam flow, and electrical energy requirements for the entire industrial plant. In order for optimization to occur, the boiler controls must be able to transmit to the extraction turbine control system the required level of extraction steam pressure and/or flow and/or megawatt output. Use of the boiler control system as a remote control system to automatically send into the extraction turbine control system all of the various process setpoints requires the provision of an extraction turbine control system capable of responding to them and moving its operational level in a bumpless fashion, without the need for operator intervention.
It can be seen that prior art extraction turbine control systems reflected control strategries which did not fully exploit the extraction turbine capabilities noted earlier. It would therefore be desirable to provide a method for selection, from multiple available control loops, a particular control loop or combination of control loops reflecting a particular control strategy or strategies. It would also be desirable to provide a simplified method of extraction turbine control to fully utilize the capabilities of the extraction turbine in meeting industrial process steam and electrical energy requirements. It would also be desirable to provide an extraction turbine control system that makes more efficient use of the extraction turbine by achieving tight control of extraction process steam requirements during various process steam extraction modes. It would also be desirable to provide an extraction turbine control system with control loops that are free from drift in calibration of circuit components, thereby reducing periodic maintenance requirements. It would also be desirable to provide an extraction turbine control system that is capable of accepting remotely generated optimization setpoint signals and adjusting its operational level in accordance therewith, without the need for operator intervention once the operator has chosen a remote mode. Such a control system would enable the realization of front-end boiler fuel cost reductions because of the smoother boiler operation associated with better and more stable extraction turbine control.